1. Field of the Invention
The present invention relates in general to compensation for artifacts produced by chrominance information leaking into the luminance channel in a color video recorder/reproducer employing alternating line recording. In particular, the chrominance signal is so processed that the artifacts for a pair of adjacent lines are 180.degree. out of phase and thus, upon viewing, appear to cancel each other due to the integrating quality of the eye.
2. Description Relative to the Prior Art
One of the most troubling aspects of color video recording is the relatively large bandwidth needed to record a standard color video signal. Even allowing for acceptable bandwidth attenuation, a minimum bandwidth recording capability of about 4.5 MHz is required. A recording system for handling a signal having information contained in frequencies from about 30 Hz to 4.5 MHz (as in a typical color video signal) is almost unthinkable from an equalization standpoint alone. For example, it is well known that the gap effect causes an increase in head response of about 6 db per octave as frequency increases until a maximum response is reached, at which point the response rapidly drops off to zero. A signal extending from 30 Hz to 4.5 MHz covers over 17 octaves; and a head designed for maximum response at 4.5 MHz will have its response down by more than 100 db at 30 Hz. Such a response range is far too great to be corrected by conventional equalization means.
One avenue of approach to the equalization problem is to obviate the necessity for recording high frequencies by narrowing the bandwidth of the recorded signal. For example, if the video signal could be made to occupy a bandwidth of 2 MHz such a signal could be recorded in a frequency range extending from 0.5 MHz to 2.5 MHz (assuming a direct recording system). Since this frequency range represents only about three octaves, the head response may be equalized in a conventional manner. Further, the highest frequency to be recorded is only 2.5 MHz thereby enabling a tape-to-head speed of one-half that of a direct recording system and one-fourth that of a FM recording system.
U.S. Pat. No. 3,255,303 discloses a method of reducing the bandwidth of the recorded video signal. The method is based upon the realization that the color video signal, as transmitted, contains more information than is absolutely necessary for an acceptable picture. The patent relates to a television signal transmission system wherein luminance information is alternately recorded with chrominance information for each line of a television picture field. The video signal thus recorded contains only about half the information content of the original video signal and can, therefore, be made to occupy a bandwidth about one half as wide as the bandwidth of the original video signal. The video signal thus recorded cannot be transmitted to a television receiver without some processing. Otherwise each line of the transmitted field would contain only luminance or chrominance information, never both, and the resultant picture would be unacceptable both with respect to monochrome and color picture quality. In an attempt to produce an acceptable picture, a video signal for transmission is reconstructed wherein a given line comprises its recorded luminance (or chrominance) information plus the chrominance (or luminance) information of the preceding line.
U.S. application Ser. No. 661,204, filed Feb. 25, 1976 disclosed a color video recorder/reproducer which, while recording all of the luminance and chrominance information of the prior art alternating line recording system described in U.S. Pat. No. 3,255,303, provided the ability to record even more luminance information (luminance information for every line) with no increase in the band-width handling requirements. The color video recorder/reproducer disclosed in that application records wide band-width luminance information alternately with narrow band-width luminance plus chrominance information. The chrominance information is frequency converted to occupy a frequency band substantially adjacent the frequency band occupied by the narrow bandwidth luminance information. The narrow bandwidth luminance plus chrominance information then occupies substantially the same bandwidth as the wide bandwidth luminance information, thereby providing maximum efficiency in the utilization of the bandwidth handling capabilities of the video recorder/reproducer. Upon playback, the frequency converted chrominance information is separated from the narrow bandwidth luminance information and converted to its originally occupied frequency band. A video signal is then reconstructed by combining the signal recorded for a given line with a complementary portion of the signal record for the preceding line.
A picture displayed by the color video recorder/reproducer of U.S. application Ser. No. 661,204 filed Feb. 25, 1976 while representing an improvement over prior art systems, is not totally satisfactory. An artifact pattern is noticeable having a frequency in the 1 MHz range. Artifacts in the 1 MHz range are particularly bothersome because, unlike higher frequency artifacts which tend to disappear or become completely unnoticeable at normal viewing distances, artifacts in the 1 MHz range are large enough to destroy the picture regardless of the viewing distance.
The source of this artifact pattern must, therefore, be located and the problem eliminated; otherwise, the color video recorder/reproducer described above would not be practical in a commercial sense. A clue to the source of the artifact pattern is the frequency of the pattern itself, about 1 MHz. The frequency is too low for the artifact partern to be caused by chrominance information which, residing on a subcarrier of 3.58 MHz, occupies a frequency band from about 3 to 4 MHz. The more likely sources of trouble are an unwanted beat frequency signal improperly filtered, a mechanical defect in the recorder itself, or an amplifier, oscillator, etc., operating in a non-linear fashion.
Although the frequency of the artifact pattern provides a clue which strongly indicates some probable locations for the source of the artifact pattern, the "clue" turns out to be a misleading one. Even though considered unlikely, it has been determined that the observed artifact pattern is the result of chrominance information which has leaked into the luminance channel. While it is not obvious how chrominance information residing on a color subcarrier of 3.58 MHz and having a frequency range from about 3 to 4 MHz can cause artifacts having a frequency of about 1 MHz, it nevertheless has happened. It has been determined that the problem originates at the point where the chrominance information, which, as described above is frequency converted to a frequency band substantially adjacent the narrow bandwidth luminance information for purpose of efficient bandwidth use, is supposedly separated from the narrow bandwidth luminance information. Due to the fact that filters necessarily possess imperfect cut-off characteristics, it is not possible to completely separate the chrominance from the luminance. The result is that frequency converted chrominance information (at a frequency of about 1 MHz) is carried along with the luminance information and, upon display, causes the observed artifact pattern.